Electrophotographic marking is a well-known, commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a charged photoreceptor with a light representation of a desired document. In response the photoreceptor discharges, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image. That toner image is then transferred from the photoreceptor onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a permanent image. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
Marking machines typically include one or more substrate feeding systems. For example, a substrate feeding system might move paper from an input tray to a transfer station. A prior art substrate feeding system 110 is illustrated in FIG. 1. A nudger roll 112 having a high friction surface 114 selectively rotates in the direction 116. The nudger roll 112 is located above a substrate stack 120 that is biased upward by a spring 122. The substrate stack is comprised of individual substrates (such as paper, Mylar sheets, or cardboard). The topmost substrate 124 advances in the direction 126 when the nudger roll rotates. The substrate feeding system 110 further includes a driven feed roll 128 having a high friction surface 129 and a retard roll 130 having a high friction surface 131 that form a nip in the path of the topmost substrate 124. The feed roll rotates in the direction 134 and drives the retard roll in the direction 136. When the topmost substrate reaches the nip the high friction surface 129 grabs the topmost substrate and advances it in the direction 126. If a second substrate is attached to the topmost substrate, such as by friction or electrostatic forces, the retard roll causes the second substrate to separate from the topmost substrate as the feed roll advances the topmost substrate. Thus, the nudger roll, feed roll, and retard roll act to feed individual substrates from the substrate stack 120 in the direction 126.
While prior art substrate feeding systems such as illustrated in FIG. 1 have been very successful they have their problems. For example, eventually the various rolls must be replaced because of wear. Thus, it is highly desirable to have rolls that are easily replaced, either by a service technician or by an end user. See, U.S. Pat. No. 5,421,569 entitled "Replaceable Feed/Retard Roll Unit," inventor Davidson, issued on Jun. 6, 1995.
Other problems with prior art substrate feeding systems include retard roll stalling and multiple sheet feeding. If sheets are sufficiently stuck together the feed-roll/retard-roll separation process may not be sufficient to separate the sheets. In that case multiple sheets may pass through a substrate feeding system. The result can be detrimental machine performance, wasted paper, and undue wear. If many sheets stick together the substrate feeding system may jam. This typically results in an unhappy end user. Furthermore, an expensive and time consuming service call may be required. As marking speeds increase these problems become more critical.
Therefore, an improved substrate feeding system characterized by fewer multiple sheet feedings and reduced staging would be beneficial. Even more beneficial would be a simple pre-shingling/anti-jamming mechanism within a substrate feeding system.